As used herein, a "semiconductor device" is a silicon chip (die) containing circuit elements. A "semiconductor device assembly" or "semiconductor device package" is a silicon chip contained within a package and connected (wired or bonded) to conductive leads which exit the package.
Heat is inevitably generated during operation of a semiconductor device, and may become destructive of the device if left unabated. The problem of heat dissipation is especially relevant in semiconductor devices that have a high lead count (e.g., high I/O) or which operate at high speeds, both of which factors contribute significantly to the generation of heat by the device.
It is generally well known to provide some sort of heat sink for semiconductor devices. Heat sinks generally include at least a heat-transferring portion positioned in close proximity to the semiconductor device (die) for efficiently extracting heat therefrom, and a heat-dissipating portion remote from the die with a large surface area for dissipating heat. The heat-dissipating portion is typically formed with a number of parallel fin layers, through which air passes to remove heat from the heat sink.
In many semiconductor device packages, notably ceramic packages, M-QUAD packages, and other "lidded" packages, the semiconductor die (device) is disposed in a cavity of the package. (Such packages are referred to hereinafter as "cavity-type" semiconductor device packages.) Heat dissipation is usually accomplished in such packages through conduction of heat via a die mounting surface of the package, such as the bottom of the cavity. A significant portion of the surface area, of the die, however, is not in direct contact with the die mounting surface of the package. Therefore, no significant amount of heat is removed from the die via these "unmounted" surfaces. While some attempts have been made to conduct heat from more than one surface of a semiconductor die, these approaches are often expensive or impractical due to mechanical and/or manufacturing difficulties such as thermally induced distortions of the die and/or heat sink, differential coefficients of expansion between the die and the heat conducting material to which it is connected, damage to delicate bond wires, critical tolerances, physical size and configuration of the heat conducting device, etc.
Approaches to "heat-sinking" which involve either direct contact or an adhesive bond between a heat-generating semiconductor die and a heat-sink structure (e.g., a metal heat-dissipating structure bonded to the die) can be particularly troublesome. The semiconductor die generally expands (thermally) at a different rate than the heat sink structure. At elevated temperatures (or at temperatures significantly different from the temperature at which the heat sink was bonded to the die), such differential rates of expansion can cause mechanical stresses which can result in the die (which is relatively brittle) cracking, resulting in complete device failure.
At least one rationale for metal (or solid) heat sinks in direct or close thermal contact with the semiconductor die involves the observation that removal of heat from the die by the heat sink limits the absolute temperature rise: of the die, thereby simultaneously limiting the degree of differential thermal expansion between the die and the heat sink. Unfortunately, however, in order to take advantage of this characteristic, it is necessary to limit the range of ambient temperatures over which the die can operate. Such differential rates of thermal expansion can also have an adverse impact on the range of storage temperatures which a semiconductor device package can endure.
As semiconductor device speeds and I/O (Input/Output) densities increase, the issue of heat dissipation from semiconductor devices becomes considerably more critical.
In addition to heat conduction problems, many modern integrated circuit components, particularly MOS and CMOS components, are susceptible to damage from high voltage discharge due to electrostatic events (e.g., static: electricity discharge). Many modern air-filled packages (i.e., the package cavity is filled with air) contain carbonaceous (carbon containing) gases after sealing. An electrostatic discharge within the cavity of the package can cause a permanent carbon "track" to be formed along the path of the discharge. This "track" can cause current leakage and can render the packaged component inoperable. As a result, such integrated circuit components often require special handling and assembly procedures to minimize the probability of electrostatic discharge prior to assembly. Further, when such components are used in "harsh" electrostatic environments, special protection circuitry must often be provided external to the package.